Genetic studies have highlighted the importance of glomerular epithelial cells (podocytes) in glomerular biology. Several familial forms of nephrotic syndrome are caused by mutations in proteins expressed exclusively by glomerular podocytes. Their interdigitating foot processes cover the exterior surface of the glomerular basement membrane (GBM) and play a key role in maintaining the integrity of the glomerular filtration barrier. A growing literature suggests that this function may be regulated by small GTPases belonging to the Rho GTPase family. In this regard, RhoA may stabilize the glomerular architecture by promoting a podocyte phenotype that inhibits proteinuria and foot process (FP) effacement. In this scenario, some basal level of RhoA activity would be beneficial. In contrast, high levels of RhoA activity induced by inflammatory processes may cause podocyte injury, perhaps by inhibiting prosurvival pathways and, in turn, enhancing podocyte apoptosis. Because of the inability of the podocyte to effectively replicate postnatally, podocyte apoptosis may cause a decrease in the number of functional podocytes, which, accumulating evidence suggests, may be a final common pathway promoting glomerulosclerosis. Based on these observations, we hypothesized that inhibition of basal Rho activity is detrimental to glomerular filtration barrier function by destabilizing the glomerular architecture and promoting FP effacement and changes in glomerular permselectivity. In contrast, high levels of Rho activity also have adverse effects on glomerular function by causing podocyte depletion. To investigate these hypotheses, two specific aims are proposed. In specific aim #1, we will investigate the role of Rho-dependent signaling cascades in promoting podocyte injury by creating transgenic (TG) mice that express a constitutively activate RhoA GTPase specifically in glomerular podocytes using an inducible promoter system. We will then determine if Rho activation causes glomerular injury by enhancing podocyte apoptosis in vivo as well as determine the signaling pathways mediating the apoptotic effect. In specific aim #2, we will determine if basal Rho activity plays an important role in maintaining the integrity of the glomerular filtration barrier by expressing a dominant negative Rho GTPase in glomerular podocytes using TG technologies and determining if RhoA inhibition under basal conditions causes a change in the podocyte phenotype that promotes proteinuria. If the proposed experiments are successful, the studies will provide important insights into the role of Rho-dependent signaling cascades in regulating podocyte function.